Generally, an oilless air compressor (also termed an air pump) provides a supply of compressed air. One configuration of an oilless air compressor includes an electric motor rotating an eccentric which, in turn, causes a piston to reciprocate up and down within a cylinder. The eccentric translates the rotary motion of the motor into a reciprocating motion for the piston. On a piston down-stroke air is pulled into the cylinder and on a piston up-stroke air is pushed out of the cylinder.
In such a design, a valve plate closes the end of the cylinder above the piston. The valve plate includes one or more inlet valves that allow air at atmospheric pressure to be pulled into the cylinder on the piston's down-stroke, but do not allow compressed air to escape to the atmosphere on the piston's up-stroke. The valve plate also includes one or more exhaust valves that allow compressed air to be pushed out of the cylinder on the piston's up-stroke but do not allow the compressed air to be pulled into the cylinder on the piston's down-stroke.
A valve plate in such an arrangement may include an outlet port leading to, for example, a pressurized air tank. On a piston up-stroke, air flows out of the outlet valves, into a chamber above the valve plate, and out of the outlet port. The arrangement of the outlet valves and the outlet port may be such that the output air flow effectively makes a 180 degree turn in the chamber, rising straight up out of the cylinder, being redirected, and exiting the outlet port in the opposite direction. This change in airflow creates turbulence and back pressure, lowering the efficiency of the compressor.
The outlet port in such a valve plate may be formed from both a threaded portion and a compression fitting made, for example, of brass. The compression fitting includes two threaded portions: one connecting with the threaded portion of the valve plate and another connecting with an outlet tube, which may be made of metal such as copper or aluminum. The tube attaches to the fitting via a compression nut and sleeve. The fitting is a discrete component which results in an increased parts count, a potential leak point, a more complex manufacturing process and greater costs.
One eccentric as used in such a compressor design includes a bearing boss, which lies outside of the axis of the motor. As the motor turns the eccentric boss moves in a circle. The boss is rotatably attached to the bottom of the piston by rotatably connecting to a piston bearing bore, a circular hole at the bottom of the piston. As the motor turns the eccentric, the piston is moved up and down. The eccentric boss is surrounded by a bearing; the bore at the bottom of the piston clamps around the bearing. The bearing reduces friction between the eccentric boss and the piston. The piston bearing bore may not be a complete circle in that a slit or small gap exists at the bottom of the piston. This slit or gap allows the bore to expand and contract slightly and allows the tension of the piston bearing bore against the bearing to be adjusted. Two clamping structures extend from the bottom of the piston, one on either side of the slit or gap, and include screw holes. A screw and bolt may be inserted into the clamping structures to alter the tension of the bore against the bearing.
The piston may be die-cast as one component. As the die-cast tool closes, molten metal is injected into the tool, and then the tool separates. The screw hole through the clamping structures extends in the direction that the tool parts separate; to create the hole a core pull is added to the die-cast tool. The use of the core pull adds to the cost of creating the piston.
Such a compressor configuration typically includes a pump frame which is attached to the motor assembly and also to the cylinder. The pump frame supports the cylinder and, since the axle of the motor extends through a bore in the pump frame to attach to the eccentric, the pump frame also helps to support the piston. A bore in the frame holds a bearing which supports the motor axle. The frame bore may include a lip on its outside edge which provides a stop for the bearing when it is pressed into the bore during manufacturing. Such a lip increases the distance between the portion of the frame supporting the axle (through the bearing) and the piston, thereby increasing the moment of force and thus increasing the stress in the frame bearing and the frame.
The spacing of the cylinder outwardly from the motor also adds to the moment and the stress on the bearing. Further, a compressor may include a pump frame with a face which is bowed outward, away from the motor. This also increases the distance between the portion of the pump frame supporting the axle and the piston.
In certain compressor designs, as the piston is forced up and down by the eccentric, the piston also wobbles, or rocks within the cylinder. Such designs may include a flexible seal, formed of a material not requiring oil lubrication, which extends around the perimeter of the piston to ensure the space above the piston is sealed as the piston rocks.
One factor reducing the life of such a piston seal is the angle of the piston during the compression stroke. When the piston is at its top dead center, the head of the piston is flat with respect to the cylinder and the surface of the cylinder head is perpendicular to the axis of the cylinder. Due to piston wobble, however, the piston head is slanted against the cylinder during both the up-stroke and the down-stroke. During the up-stroke, in which air is compressed, the piston seal is pressed unevenly against the cylinder, causing excessive wear of the piston seal.
As air is compressed by the piston, it is heated. The heated air heats components of the air compressor, causing faster wear and reducing operating efficiency. An important factor contributing to piston seal wear is its operating temperature; as the operating temperature increases the life of the seal decreases. To reduce the temperature of such pumps a cooling fan may be included. Due to the location of the fan and the arrangement of the components of certain compressors, such a cooling fan may blow air in a direction more or less perpendicular to the axis of the cylinder. Such an air flow arrangement, however, cools the cylinder inefficiently. The fan is typically connected directly to the eccentric boss and thus rotates at the same speed as the motor. While the compressor and motor may operate at different speeds, the fan may be most efficient at only one speed.
One technique for reducing heat in air compressors is described in U.S. Pat. No. 5,937,736 to Charpie. Charpie describes a piston cap having cooling fins. The piston cap is secured to the piston head, and the cooling fins extend through holes on the piston head. Such a solution is imperfect, as heat is effectively removed only from the piston head. While the cap is secured to the head, heat does not transfer effectively across gaps in metal, and thus the piston head and the piston rod (which is integral with the piston head) are not cooled by the heat sink action of the fins. The size, shape and number of the holes in the piston head limit the size, shape and number of the cooling fins. Further, a piston head with holes for cooling fins may be harder or more costly to manufacture. It is desirable to have a more efficient means for cooling a piston that also allows for easier and less costly construction.
As discussed, when the piston is at its top dead center, the head of the piston is flat with respect to the cylinder, and the surface of the cylinder head is perpendicular to the axis of the cylinder. As the piston tilts away from top dead center, one edge of the piston head rises higher than the center of the piston head. Thus a clearance volume must be provided between the top of the piston and the valve plate. This clearance volume results in a dead space above the piston, reducing the efficiency of the compressor and increasing the heat levels in the compressor.
It is desirable to have an air compressor which overcomes at least some or all of the aforementioned shortcomings of known air compressors.